America's youngest billionaire is eight days younger than his former Harvard roommate Mark Zuckerberg. Facebook's first chief technology officer, he left in 2008 to start Asana, a software company that allows individuals and small companies to better collaborate. For now, his entire fortune comes from his 6% stake in Facebook.

The Harvard dropout and Facebook CEO was the biggest percentage gainer on this year's Forbes 400 list and the unwitting star of box-office hit The Social Network. Last month he agreed to donate $100 million to Newark's troubled schools.

The son of the late Dan Duncan assumed control of part of his family's $12.4 billion pipeline empire after his father's death last March. Their energy company Enterprise Products owns more than 49,000 miles of pipelines.

Brazilian-born Saverin cofounded Facebook with Harvard classmate Mark Zuckerberg and for a brief time had a one-third stake. When Zuckerberg quit school to relocate to California, Saverin stayed behind to graduate. A year later Facebook sued him; he countersued. The parties settled with Saverin apparently getting a 5% stake and cofounder bio on Facebook's site.

The former Enron oil trader founded hedge fund Centaurus after the energy outfit famously collapsed. His fund now has $5 billion under management, and he and his wife Laura recently pledged half of their wealth to the Gates-Buffet challenge.

Emigrated from Russia at age 6; his mother was a research scientist at NASA. He met Larry Page in computer science Ph.D. program at Stanford and dropped out in 1998 to start Google. Brin now focuses on raising margins with Instant Search and building new businesses in communications, and invests in space travel initiatives and Parkinson's research on the side.

The Google cofounder dropped out of his Stanford Ph.D. program in 1998 to start the search engine. Nowadays his personal passions include buying up chunks of residential Palo Alto for a network of houses that use new types of fuel cells, geothermal energy and rainwater capture. He also rides a Zero X electric dirt bike and an electric sports car from Tesla Motors, in which he and Sergey Brin are investors. Page is a board member of the X Prize Foundation, a nonprofit looking for breakthroughs in genomics, energy and space exploration.

The youngest son of the late William Ziff Jr. who built the Ziff-Davis publishing empire (PC Magazine, Car & Driver, Boating) and later sold out. Daniel and his brothers Dirk and Robert inherited their father's fortune, which they have since reinvested in Ziff Brothers Investments. He recently got married.

Born in Taiwan, Yang moved to the U.S. at age 10. He created his Web directory in 1994 with partner David Filo when both were Stanford grad students. Yahoo cofounder made his debut among America's 400 richest at age 29. Still with Yahoo, but no longer its head honcho. He sits on other boards and makes a number of charitable donations.

Griffin began investing as a Harvard undergrad after he read a Forbes article about the Home Shopping Network. Nowadays his hedge fund Citadel, which has seen assets drop to $11 billion from $20 billion in recent years, is diversifying; it recently launched new funds in equities and mortgages and is jump-starting an investment banking business.

Daughter of the late Dan Duncan assumed control of part of the family's $12.4 billion pipeline empire after his death last March. Her father started oil giant Enterprise Products with two trucks in 1968.

Thiel cofounded PayPal in 1998 and sold it to eBay in 2002 for $1.5 billion, personally netting $60 million from the sale. He then invested $500,000 in Facebook in 2004; this stake now makes up the bulk of his net worth. The former national chess champion co-produced the film Thank You for Smoking.

French-born Iranian moved to the U.S. with his family at age 6. He wrote code for eBay at age 28 and became a billionaire in 1998 when the company went public. An active and eclectic donor, his Omidyar Network has given $5.5 million to a company designing solar powered lanterns and $500,000 to a political satire show in Kenya.

Filo met cofounder Jerry Yang as a Stanford grad student; he is still on leave from the Ph.D. program they left in 1995 to form Yahoo. The highly regarded and liked engineer has sold little stock over the years; whatever has been sold lately has apparently gone to charity.

His late father sold 95% of the family's Ziff-Davis publishing empire to Forstmann Little for $1.4 billion in 1994. Along with brothers Dirk and Daniel, Robert reinvested the proceeds into Ziff Brothers Investments.

The U. of Maryland dropout made his first million at age 20 by selling advertising at colleges. His marketing firm, Snyder Communications, was sold to Havas for $2 billion in stock. He bought the Redskins in 1999 for $800 million; the team is now worth $1.55 billion. Through Red Zone Capital he also owns big stakes in Johnny Rockets restaurant chain and Dick Clark Productions.

Harris cofounded private-equity firm Apollo Management in 1990 with Leon Black and Marc Rowan after their former employer, Drexel Burnham Lambert, collapsed. Next Apollo wants to raise $50 million in its own initial public offering. Harris is also a trustee at Mount Sinai Hospital.

Grandchild of A.N. Pritzker, who created the industrial conglomerate Marmon and hotel chain Hyatt. Pritzker co-owns venture fund, New World Ventures, and private holding company, Pritzker Group, with his brother Tony.

Started selling computers from his college dorm room in 1984; took Dell public in 1988. Made his debut on the Forbes 400 at age 26; still one of the youngest list members nearly two decades later. Dell stepped down as CEO in 2004 but returned to the post in 2007. His firm completed purchase of Perot Systems for roughly $3.9 billion in November 2009.